Conventional binoculars are focused on objects at different distances by rotating a round-knob or a focus wheel which adjusts the position of one or more ocular lenses until the magnified image appears sharp. Recently, focus adjusting devices such as the one shown in U.S. Pat. No. 4,066,329 to Van Exel, which is hereby incorporated by reference, have been developed utilizing a lever-like adjusting knob. More particularly, the Van Exel adjusting knob is a treadle-like "rocker" knob having platform surfaces which fall naturally under the user's fingertips to enable the user to quickly adjust the lenses of the binocular.
The Van Exel patent, along with U.S. Pat. No. 3,540,792 to Akin, solved a subtle problem with conventional round-knob focus adjustments which occurs from the inherent tendency of the human eye to accommodate itself to an out-of-focus image. Precise focus is difficult to achieve in a single adjustment, due to the inherent tendency of the eye to "force" a slightly unfocused image into focus. This action arises from an unconscious distortion of the eye's crystalline lens to achieve sharpness of the retinal image. The eye's crystalline lens will "lead" a slow focus adjustment and an accurate adjustment is not accomplished until the binocular has been removed from and returned to the eye several times.
Both the Van Exel and Akin patents provide solutions to this problem by providing a focus adjustment capable of rapid actuation such that true focus is achieved before the accommodation tendency can manifest itself. Due to the ability to rapidly focus objects into view, this type of adjusting knob has become very popular among bird enthusiasts and for military surveillance purposes. A problem experienced, however, is that the focusing knob may be inadvertently displaced by contacting the body or clothing, or simply by nervous twiddling of the lever. Frequently, the object in view is lost in the split second it takes for readjustment.
The solution to this problem is to provide an automatically self-locking capability to the focus adjustment mechanism such that the user's ability to rapidly readjust the focus adjustment is not affected.